Heat exchangers are necessary components of many industrial processes. A common form of heat exchanger is a shell through which a large number of tubes pass, the shell enclosing the fluid to be cooled and the tubes conducting a coolant for removing the heat. Such heat exchangers are commonly used in refining, petrochemical and power generation industries. In particular, in steam power plants, the steam condenser employs cooling water passing through many thousands of heat exchanger tubes. After steam has passed through a turbine, it is condensed in the shell of the heat exchanger. The efficiency of heat removal from the steam by the cooling water determines the back pressure at the turbine exhaust, and this pressure significantly affects the total energy extracted from the steam. Energy lost by not extracting it from the steam leads directly to an increase in power generation costs.
To maintain maximum heat transfer efficiency in a tube and shell heat exchanger, it is necessary to minimize buildup of film, such as bio-growth or a chemical scale, on the internal surface of the tubes of the heat exchanger. Chemical and mechanical treatments are used to inhibit the formation of or remove such films. On-line and off-line mechanical techniques have been used for removing such film buildups. On-line techniques, in which the heat exchanger is not taken out of service as the tubes are cleaned are the subject of the present invention.
There are several types of cooling water systems. In "once-through" systems, the cooling water passes through the heat exchange system only once. The warmed water is discharged--often to its original source; a river, lake, well or the ocean. In closed recirculating systems, the water is completely confined within the steam pipes and heat exchangers. The heat is generally dissipated by heat exchange with air. In open recirculating cooling tower systems, the water is continuously reused, but the system is open to the atmosphere in a cooling tower.
Cooling towers, a component of many cooling water systems are designed in many different configurations. Common to all is a means for forming a large surface area between cooling water and the air. Air is drawn through the dispersed liquid in a horizontal direction in cross-flow cooling towers and in a vertical direction in counterflow cooling towers. The air may be driven by natural draft and a large chimney, such as the hyperbolic towers commonly associated with nuclear power plants, or by mechanical drivers-normally fans driven by electric motors. In a cooling tower, the water is pumped to a level where it falls by gravity, either through orifices or spray nozzles, and creates droplets or a splash zone. Means are provided for dispersing the water over the area of the "fill" through which it will fall. This dispersal may be through conduits into a pan, or open basin, or through a flume.
Cleaning of the tubes of heat exchangers on-line by pumping sized solid bodies through the tubes is known in the art. U.S. Pat. No. 2,801,824 describes a system wherein deformable spheres made of a foamed elastomer are pumped through the tubes and recovered down steam of the heat exchanger. There is a considerable body of art, such as, for example, that described in U.S. Pat. No. 4,830,099 related to apparatus for removing such deformable cleaning bodies from cooling water downstream from the heat exchanger. These devices involve screens in a particular configuration which separate the deformable cleaning bodies from the flow, with the cleaners removed from the screens by additional suction means. Such devices are used as an integral part of a conduit or closed flow stream immediately downstream of the heat exchanger.
In recent years, a new type of on-line tube cleaner has been described, which is called a "hard body"-type cleaner. These cleaners have a body which is generally spheroidal in shape and made of a material such as polypropylene plastic. Attached to the body is a flexible disk of plastic such as polyurethane which is designed to wipe and clean the inside wall of a heat exchanger tube as the body is pumped through the tube. Hard-body tube cleaners are disclosed in U.S. Pat. Nos. 4,473,787 and 4,569,097.
U.S. Pat. No. 4,696,318 discloses a system which can be used to remove "hard-body" type (i.e., non-deformable) cleaning bodies from open recirculating cooling water systems. The removal is accomplished by adjusting the density of the cleaning bodies such that they float and then recovering them by skimming from a stream downstream of the steam condenser of a power plant. A stream suitable for recovering hard-body cleaners utilizing the floatation method of recovery is not available in many power plants. In some plants, excess turbulence in the stream prevents successful recovery by flotation.
Apparatus for recovering floating, hard-body type cleaning elements in open water surfaces available in or around some cooling water systems is disclosed in U.S. Pat. No. 5,647,428.